This drops support for type hierarchy instance creators. I don't expect this to be a problem. We'll also detect fewer errors where multiple type adapters can serialize the same type. With APIs like getNextTypeAdapter, I think this might actually be an improvement!
The motivation for this change is to give tree-style adapters precedence order in registration. This fixes the test I committed earlier today, where registration order was not honored.
This renamed ParameterizedTypeHandlerMap to the shorter 'TypeMap'. For type adapters, this is now only used for type hierarchy. We still need non-hierarchy support in TypeMap for instance creators; I'll be looking for workarounds to see if further simplification is possible here.
